So the driver should go to jail for six years (mind you, that's more than half of how long you've been alive) because of some poor footwork and then human nature kicking in against his better judgement? I'm glad it's not people like you who populate legislature, it's absurd that someone should lose six loving years of their life because of a mistake and poor judgement after the fact when every bit of their instincts said RUN AND HIDE.
Even if he had killed the kid, vehicular manslaughter charges are no where near that intense. In California and Georgia, it's a maximum of 1 year as long as you weren't intoxicated and didn't intend on killing them. I don't know about other states, but I'm too lazy to look 'em up and that seems to be the norm. In CA, a hit in run is a misdemeanor punishable by up to 6 months in jail. So, if this crime happened in CA, it'd be a maximum of 1.5 years in prison. Not loving six.
(I picked CA because it's the most populous state, with an 11% chance that OP lives there)
It wasnt instinct. He made the decision to run away. Instinct/nature makes it sound like he couldn't help it to run away from the scene.
By jail time, I believe it should be anywhere from one year to ten years (tons of deciding factors though, including manner, care, intoxication if applicable, cooperation, etc). If he stayed at the scene, I think that a simple fine and paying the victim's medical bill would be enough to go free.